Polyomavirus normally establishes a silent, persistent infection in its natural host, the mouse, but under experimental conditions can cause widespread lytic infection and induce tumors in up to a dozen different tissues, including a high frequency of tumors in thymic tissue, salivary glands and mammary glands. Although the virus has been studied extensively in cell culture, little is known about its interaction with host cells in the animal. The experiments in this proposal are designed to explore the virus-cell interactions that result in tissue specificity of tumor formation by polyomavirus. The two specific aims in this proposal are based on the hypothesis that viral regulatory sequences interacting with cellular factors are responsible for tissue specific tumor induction. The first goal is to define the viral genetic determinants responsible for tissue specific tumorigenesis by evaluating the tumor profile of viruses containing mutations in potential regulatory sequences. These regulatory regions include a 4Obp duplication of sequences on the early side of the origin of replication that we have previously shown to be necessary for the induction of thymic epitheliomas, and the polyoma enhancer sequences that are important for virus regulation in cell culture systems. Our second goal is to use these regulatory sequences to identify and characterize the cellular factors that bind to them. The DNA binding proteins will be identified by gel shift and footprinting experiments and further characterized by UV crosslinking experiments and affinity chromatography. Polyomavirus gene expression and replication, as well as the oncogenic pathways triggered by the virus, utilize cellular factors that are normally involved in signal transduction and cell growth control, and that are altered in many cancers of non-viral etiology. Thus, understanding tumor induction by polyomavirus at the molecular level and identifying cis-acting sequences and cellular factors involved in the tissue specificity of tumor formation will provide new information relevant to the fields of cell growth control and cancer.